Short pulse microwave source



May 14, 196s D. c. FORsTr-:R

SHORT PULSE MICROWAVE SOURCE 2 Sheets-Sheet l Filed May l5, 1967 Z. l l M7 Z WNNW @s ww mv w 4 n\ l I l l- .xA .1- \m\.\\ m f Wg. a IRAQ H mx NTI m MMNkN. u Gm NArl A A l\ n .l n m `wN\ m M. Nm N m MJ mw m Q mmmmm i mm [fwn n l w u x www mN\ NN m N\ ww X W NTE. w qm May 14, 1968 D. c. FORSTER 3,383,620

SHORT PULSE MICROWAVE SOURCE Filed May l5, 1967 2 Sheets-Sheet 2 United States Patent 3,383,620 SH'DRT PULSE MICROWAVE SOURCE Donald C. Forster, Woodland Hills, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed May 1S, 1967, Ser. No. 638,371 4 Claims. (Cl. 331-82) ABSTRACT OF THE DISCLOSURE This disclosed microwave pulse source includes a slowwave structure of length L disposed about a stream of electrons traveling with a velocity substantially synchronous with the phase velocity vp of microwaves propagating along the slow-wave structure. The ends of the slow-wave structure are electrically short cir-cuited to provide a resonant electromagnetic system, the quality factor Q of which is varied at a frequency either by amplitude modulating the electron stream or by varying the energy dissipation in a lossy element adjacent the slow-wave structure by means of a magnetic eld controlled ferrite device.

This invention relates generally to microwave devices, and more particularly relates to a pulse generator for providing short duration microwave pulses.

It is an object of the invention to provide a pulse source capable of gcneratino microwave pulses having durations about an order of magnitude shorter than those afforded by microwave pulse generators of the prior art.

A microwave pulse generator according to the invention includes an electron gun for launching a stream of electrons along a predetermined path and a slow-wave structure of preselected length disposed along and about the electron stream path for propagating electromagnetic wave energy with a predetermined phase velocity substantially synchronous with the velocity of the electron stream. An electromagnetic wave reflecting surface is provided at each end of the slow-wave structure so as to form a resonant electromagnetic system. The quality factor Q of the electromagnetic system is varied at a frequency essentially equal to one-half of the predetermined phase velocity divided by the preselected slow-wave structure length so that oscillations may be sustained at a plurality of frequencies separated by the Q variation frequency. Signal components at these various oscillation frequencies combine to produce a microwave pulse of extremely short duration.

In one embodiment of the invention the Q of the resonant electromagnetic system is effectively varied by modulating the electron stream at the desired frequency, In another embodiment of the invention `the Q variation is afforded by varying the energy `dissipation in the syste-m at the desired frequency by means of an electric eld rotating ferrite device to which a varying magnetic eld is applied.

Additional objects, advantages and characteristic features of the invention will become readily apparent from the following detailed description of preferred embodiments of the invention when considered in conjunction with the accompanying drawing in which:

FIG. 1 is a longitudinal sectional view of a microwave pulse generator in accordance with one embodiment of the invention;

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is a frequency-phase (tv-) diagram used in explaining the operation of the present invention;

FIG. 4 is a diagram illustrating the power output frequency spectrum achievable with a generator according to the invention;

FIG. 5 is a diagram illustrating output microwave pulses from a generator according to the invention as a function of time;

FIG. 6 is a longitudinal sectional view of a microwave pulse generator in accordance with an alternate embodi ment of the invention; and

FIG. 7 is a sectional view taken along line 7-7 of FIG. 6.

Referring to FIG. 1 with greater particularity, a microwave pulse source in accordance with the invention may be seen to include a slow-wave structure 10 of preselected length L for propagating microwave energy in a serpentine path about an electron stream traveling longitudinally through the slow-wave structure 10. The electron stream is generated by a conventional electron gun 12 disposed at one end of the slow-wave structure 10 and which gun includes an electron emitting cathode 14 and larnentary heater 16. Voltage sources 18 and 20 may be connected as shown to provide operating potentials for the heater 16 and cathode 14. Disposed between the electron gun 12 and the slow-wave structure 10 is an accelerating anode 22 to which is applied a D-C voltage Va, generated by a voltage source 24, and a modulating A-C voltage Vm sin 21rAft generated by an A-C source 26. The modulating voltage will be discussed in greater detail below.

In a microwave pulse source according to the invention any of a variety of different types of circuits may be used for the slow-wave structure 10. Such circuits include but are not limited to the helix circuit, the cloverleaf circuit, the ring-bar circuit, the vane line circuit and the coupled cavity circuit. However, for purpose of illustration, the microwave pulse generator in accordance with the einbodiment of FIG. l is shown to have a coupled cavity type of slow-wave circuit. This circuit comprises a series of circular plates 28 which are mounted in a cylindrical waveguide 30 in planes perpendicular to the longitudinal axis of the waveguide 30 at spaced intervals along the waveguide 30 to provide a series of interaction cells, or cavities, 32. The plates 28 and the waveguide 30 may be made of an electrically conductive material, such as copper or brass, for example. Each cavity 32 is defined by the walls of the two adjacent plates 28 and that portion of the inner surface of the waveguide 30 located between these two plates. The size of the cavities 32 determines the frequency of the microwave energy capable of propagating along the slow-wave structure 10, while the number of cavities per unit length of slow-wave structure determines the phase velocity vD (ratio of frequency w to phase ,8) of the propogating microwaves.

Adjacent cavities 32 are interconnected by means of an off-center coupling hole 34 provided through the intermediate plate 28 to permit the transfer of microwave energy `from cell to cell along the slow-wave structure 10. As is illustrated, the coupling holes 34 may be substantially kidney-shaped and may be alternately disposed apart with respect to the longitudinal axis of the slowwave structure, although the coupling holes 34 may -be of other shapes and may Ibe staggered in other arrangements well known in the art. Each plate 28 also defines a drift tube, or ferrule, 36 in its central region. The drift tube 36 is in the form of a cylindrical extension, or lip, protruding axially along the path of the electron stream from both surfaces of the plate 28, i.e. in both directions normal to the plane of the plate 28. The drift tubes 36 are provided with central and axially aligned apertures 38 to provide a passage for the flow of the electron stream. Adjacent ones of the drift tubes 36 are separated by an interaction gap 4l) in which energy exchange between the electron stream and the microwave energy propagating along the slow-wave structure 10 occurs.

The two plates 23' disposed at the respective ends of the slow-wave structure 1t) are not provided with any coupling holes 34, but rather define continuous planar surfaces except for the ferrule and electron stream aperture. Thus, the plates 23 provided at each end of the slow-wave structure 10 an electromagnetic wave reflecting surface which functions electrically as a short circuit.

Disposed at the end of the slov-Wave structure 10 remote from the electron gun 12 is a collector electrode 44 which intercepts the stream electrons and dissipates their kinetic energy. In order to constrain the electrons into a narrow, well-collimated path through the ferrules 36 a longitudinal focusing magnetic field is provided, for example by means of a solenoid d6 which is disposed concentrically about and is substantially coextensive with the slow-wave structure 10. A waveguide 48 is connected to the slow-wave structure 10 in order to convey microwave output pulses from the generator to external circuitry (not shown).

The slow-wave structure 10 is biased from a source of potential 50 to a predetermined potential VD which causes the electron stream to travel with a velocity no in accordance with the relationship where e is the electronic charge and m is the mass of an electron.

The curve 52 of FIG. 3 illustrates the dispersion characteristic (frequency w as a function of phase for electromagnetic waves capable of propagating along the slowwave structure 10. The slope of the curve S2 designates the phase velocity vp of the traveling waves, and it may be seen from FIG. 3 that the curve 52 possesses a substantial linear region wherein the phase velocity vp is constant. The voltage VD furnished by the source 5t) is selected to provide an electron beam velocity no, illustrated by the dashed curve S in FIG. 3, which is substantially synchronous (coincident) with the wave phase velocity vp throughout the linear region of the curve 52. Throughout this region of synchronism energy exchange between electrons of the stream and the traveling microwaves can occur.

The slow-wave structure functions not only as a Wave propagating interaction structure, but on account of its wave retiecting end surfaces 29 also functions as an electrical resonator which is capable of sustaining an oscillatory mode at each wavelength for which its length L is an integral multiple of M2. The frequency separation Af between these oscillatory modes is given by Un Afz 2) Thus, it is possible for oscillations to occur at a plurality of resonant frequencies f1 fn each separated from the next successive resonant frequency by the amount Af in accordance with Equation 2.

As has been mentioned above, the A-C source 26 applies a modulating voltage Vm sin 21u/ft, where Af is selected to satisfy Equation 2, to the anode 22 so that the electron stream is amplitude modulated at the frequency Af. As may be seen from FIG. 3, since the phase velocity vp of the microwaves traveling along the slow-wave structure 10 is substantially synchronous with the electron stream velocity no at each of the frequencies w1 wn (where w=21rf), energy exchange between the electrons and the microwaves can occur at each of these frequencies. Thus, oscillations are set up in the resonant slow-wave structure 10 at each of the frequencies f1 fw producing the power output frequency spectrum illutsrated in FlG. 4. By Fourier analysis this spectrum of irequencies f1 L, can be shown to produce a substani tially rectangular pulse having a duration f which may be approximated by fn-f1 (55) Thus, the generator of FIG. l produces output microwave pulses 60, shown in FIG. 5, of a duration -r and occurring at a pulse repetition frequency Af.

As a specific illustrative example of a microwave pulse source which may be constructed in accordance with the embodiment of FIGS. 1-2, the slow-wave structure 1t) may have a 10 gc. bandwidth at frequencies centered around 94 gc. with a typical modulating frequency Af of mc. The resulting pulse duration T would be For a helix slow-wave structure having a 2 gc. bandwidth at X-band frequencies, the resulting pulse duration would be =0.l nanosecond In the embodiment of FIGS. 1-2, the various oscillatory modes f1 fn are set up by modulating the electron stream at the frequency Af. However, other schemes may be employed to produce the desired oscillatory modes, and another embodiment of the present invention in which a different approach is utilized to produce oscillations at the desired frequencies is illustrated in FIGS. 6 and 7. The embodiment of FIGS. 6 and 7 is similar to the embodiment of FIGS. l and 2, and hence components in the embodiment of FiGS. 6 and 7 are designated by the same reference numerals as corresponding components in the embodiment of FIGS. 1-2 except for the addition of prefix numeral 1. However, in the embodiment of FIGS. 6-7 a rectangular waveguide 170 is coupled to the interaction cavity 132 adjacent the collector end of the slow-wave structure 110, and a ferrite rod 172 is longitudinally mounted in the center of the waveguide 170 by means of a support 174 of low loss dielectric material. An elongated energy absorbing element 176 of a lossy material such as Kanthal is mounted across the waveguide 170 parallel to its broad walls at its end adjacent the slow-wave structure 110. An A-C voltage source 126, which generates a modulating voltage Vm sin 21rAft is connected across a coil 180 which is wound about the waveguide 17 0.

In the operation of the embodiment of FlGS. 6-7, electromagnetic wave energy at the collector end of the slow-wave structure enters the waveguide 170 with its electric field E parallel to the narrow walls of the waveguide 170, i.e. perpendicular to the plane of the paper (FIG. 6). When the voltage Vm from the source 126 is zero, there is no current flow through the coil 13'!) and no magnetic field is applied to the ferrite rod 172. Thus, the electromagnetic energy traveling along the waveguide 170, is reflected by the end Wall of the waveguide 170, and returns to the slow-wave structure 110 without having the orientation of its electric field E rotated when passing through the ferrite rod 172. Hence, the electromagnetic energy traverses the lossy element 176 with its electric field perpendicular to the length of the element 176, and little electromagnetic energy is dissipated in the elcment 176.

On the other hand, when the voltage Vm from the source 126 is not zero, the resultant current flow through the coil 180 establishes a magnetic field along the longitudinal axis of the ferrite rod 172. The electric eld E of the electromagnetic energy traveling along the waveguide now undergoes a 45 rotation with each passage through the ferrite rod 172, and thus a net electric field rotation of 90 is experienced as the electromagnetic energy returns to the slow-wave structure 110 after a forward and backward traversal ofthe waveguide 17d. Since the electric field E of the electromagnetic energy return- =0.5 nanosccond ing to the slow-wave structure 110 lies parallel to the length of the lossy element 17e, substantial electromagnetic energy is dissipated in the element 176. This energy dissipation varies as a function of time in accordance with the voltage Vm sin ZTrAft from the source 12:6.

lt will be appreciated that regardless of whether the loading on the resonant electromagnetic system including the slowl ave structure 1d and the electron stream traveling therethrough is changed by modulating the electron stream at the desired frequency or whether the electromagnetic energy dissipation in an auxiliary lossy element is varied at this frequency, the net result is that the energy absorption properties (hence the quality factor Q) of the system are effectively varied at the desired frequency of. As a result of this variation in the Q of the system, oscillations at the various frequencies f1 fn are set up, enabling extremely short microwave output pulses to be produced. Thus, regardless of the specific modulating arrangement, the present invention involves effectively varying the quality factor Q of the resonant electromagnetic system at a frequency Af essentially equal to onehalf of the phase velocity vp divided by the resonator length L.

Accordingly, although the invention has been shown and described with reference to particular embodiments, various changes and modifications obvious to a person skilled in the art to which the invention pertains are deemed to lie within the spirit, scope and contemplation of the invention.

What is claimed is:

1. A microwave pulse generator comprising: means for providing a stream of electrons along a predetermined path, slow-wave structure means of preselected length disposed along and about said path for propagating electromagnetic wave energy with a predetermined phase velocity substantially synchronous with the velocity of said electron stream, means for providing an electromagnetic wave reflecting surface at each end of said slow-wave structure means whereby a resonant electromagnetic system is provided, means for effectively varying the quality factor Q of said electromagnetic system at a frequency essentially equal to one-half of said predetermined phase velocity divided by said preselected length, and means for extracting microwave output pulses from said slow-Wave structure means.

2. A microwave pulse generator comprising: means for providing a stream of electrons along a predetermined path, slow-wave structure means of preselected length disposed along and about said path for propagating electromagnetic wave energy with a predetermined phase velocity substantially synchronous with the velocity of said electron stream, means for providing an electromagnetic wave reflecting surface at each end of said slow-wave structure means whereby a resonant electromagnetic system is provided, means for effectively varying the energy absorption properties of said electromagnetic system at a frequency essentially equal to one-half of said predetermined phase velocity divided by said preselected length, and means for extracting microwave output pulses from said slow-wave structure means.

3. A microwave pulse generator comprising: means for providing a stream of electrons along a predetermined path, slow-wave structure means of preselected length disposed along and about said path for propagating electromagnetic wave energy with a predetermined phase velocity substantially synchronous with the velocity of said electron stream, means for providing an electromagnetic wave reflecting surface at each end of said slowwave structure means, means for modulating said electron stream at a frequency essentially equal to one-half of said predetermined phase velocity divided by said preselected length, and means for extracting microwave output pulses from said slow-wave structure means.

'4. A microwave pulse generator comprising: means for providing a stream of electrons along a predetermined path, slow-wave structure means of preselected length disposed along and about said path for propagating electromagnetic wave energy with a predetermined phase velocity substantially synchronous with the velocity of said electron stream, means for providing an electromagnetic wave reecting surface at each end of said slowwave structure means, a waveguide coupled to said slowwave structure means adjacent the wave reflecting surface at one end thereof, an elongated energy absorbing element mounted across said waveguide at its end adjacent said slow-wave structure means, said energy absorbing element being disposed parallel to the broad walls of said waveguide, a ferrite element mounted in said waveguide parallel to the longitudial direction of said waveguide, means for applying a varying magnetic field to said waveguide at a frequency essentially equal to one-half of said predetermined phase velocity divided by said preselected length, and means for extracting microwave output pulses from said slow-wave structure means.

References Cited UNITED STATES PATENTS 3,181,024 4/1965 Sensiper 331-82 X 3,221,205 1l/1965 Sensiper 331-82 X ROY LAKE, Primary Examiner.

S. H. GRIMM, Assistant Examiner'. 

